Green roof substrate physical properties differ between standard laboratory tests due to differences in compaction

Abstract

Green roofs are expanding internationally due to the well documented benefits they provide for buildings and cities. This requires transferable knowledge of the technological aspects influencing green roof design, particularly substrate properties. However, this is made difficult due to differences in substrate testing methods referred to in green roof guidelines and standards. Therefore, we tested a green roof substrate using laboratory-based methods from European (FLL), North American (ASTM) and Australian (AS) green roof guidelines and standards to determine how these methods vary in characterising substrate physical properties (bulk density, water permeability and water holding capacity at field capacity (WHC)). Further, we compared the results from the laboratory-based methods with measures of bulk density and WHC in green roof platforms to determine whether standard methods accurately represent substrate properties in-situ. Results from the standard test methods varied due to differences in sample compaction. The standard test methods that employ Proctor hammer compaction (FLL and ASTM) had greater bulk density (at field capacity and dry) and lower water permeability than Australian standard methods that employ free-fall compaction. WHC did not differ among the standard methods. The Australian standard method better reflected bulk density at field capacity and WHC of the substrate under in-situ green roof conditions. For mineral based substrates, our results suggest that for the FLL and ASTM testing methods, a single Proctor hammer drop will produce a degree of sample compaction equivalent to the free-fall method (AS) and be more representative of bulk density in-situ. Subtle changes in testing procedures would allow for more direct comparison of substrate properties between standard methods and help enable the international transfer of knowledge for substrate design.

Keywords: Green roof; Guidelines; Physical properties; Standards; Substrate; Water holding capacity.